Resin transfer molding (RTM) is a process used to fabricate fiber reinforced composite articles. The process involves two basic procedures, fabricating a fiber preform in the shape of the finished article and impregnating the preform with a thermosetting resin. The resulting fiber reinforced composite article displays high strength and low weight. Such articles are commonly used in the aerospace industry and for other applications which require low weight, high strength materials.
The first procedure in the RTM process is to fabricate a fiber preform in the shape of the desired article. The preform generally comprises a plurality of fabric plies which will impart the desired reinforcing properties to the finished article. For example, the fabric plies may comprise graphite or KEVLAR TM fibers. Several steps are necessary to fabricate the preform. The exact steps which are required and the optimum order for performing the steps will vary depending on the shape of the preform. Any series of steps performed in any logical order which will produce the desired preform are satisfactory. Generally, the techniques used to form the preform are borrowed from the garment industry. For example, the fabric plies may be cut according to a predetermined pattern, laid up on a mandrel having the desired shape, and loosely bonded or stabilized so that the plies maintain the desired shape after being removed from the mandrel.
Once the fiber preform has been fabricated, it is placed into a transfer mold for the second procedure in the RTM process. The mold is closed and a resin, typically an epoxy such as a TACTIX 123/H41 (TM) resin (Dow Chemicals, Midland, Mich.) is injected under pressure to wet the preform. The resin initially has a viscosity about that of water to enable it to thoroughly impregnate the preform. The temperature of the mold is then increased, causing the resin to increase in viscosity and ultimately, to solidify. A finished fiber reinforced composite article typically comprises between approximately 50 vol % and approximately 60 vol % fibers. Typically high performance aerospace composite articles comprise between approximately 55 vol. % and 60 vol. % fibers.
One of the key steps in the RTM process is stabilizing the preform prior to placing it in the mold. Stabilization is necessary to ensure that the fabric plies maintain the desired shape and orientation until they are impregnated with resin. Stabilization also prevents the fabric plies from unraveling along cut edges. A considerable amount of developmental effort has been expended to optimize the stabilization step. The most common methods used to stabilize the preform are stitching and hot iron tacking. Both of these methods are labor and time intensive.
Stitching can be a very effective means of stabilizing the preform, however, stitching is difficult to perform after the fabric plies have been laid up on a mandrel. Moreover, stitching can make it difficult to fabricate a preform into a complex shape because stitching tends to make the fabric plies too rigid to bend easily. Therefore, stitching may not be an appropriate method of stabilizing a preform which has a complex shape.
Tacking is more conducive to fabricating preforms which have a complex shape. However, it may be more labor intensive and time consuming than stitching if the preform has many fabric plies. A thermoplastic polymer is disposed between each of the fabric plies and is melted with a hot iron. The polymer is allowed to refreeze to bond the plies together. Tacking must be done one ply at a time because heat transfer is by conduction only. Tacking one ply at a time is a relatively slow process because most of the reinforcing fabrics are poor heat conductors. Such a technique is taught by commonly owned U.S. Pat. No. 4,470,862 to More et al.
Accordingly, there has been a continuous effort in this field of art to develop a less labor and time intensive method of stabilizing RTM preforms.